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										01-Java基础
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										04. 反射和动态代理
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										02-Java并发
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										01. 多线程基础
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										02. CAS
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										03. synchronized
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										04. AQS和ReentrantLock
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										05. 线程池
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										06. ThreadLocal
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										07. CountDownLatch和CyclicBarrier
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										08. 为什么需要Java内存模型
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										09. 深入浅出Java内存模型
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										03-Java容器
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										01. List集合
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										02. Map集合
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										04-Java虚拟机
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										01. Java编译到执行的过程
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										02. 双亲委派机制
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										03. JVM内存结构
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										04. 垃圾回收机制
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										05. CMS垃圾收集器
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										06. G1垃圾收集器
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										07. JVM调优
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										05-Spring
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										01. SpringMVC
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										02. Spring基础
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										03. SpringBean生命周期
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										06-Redis
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										01. Redis基础
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										02. Redis持久化
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										03. Redis主从架构
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										04. Redis分片集群
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										07-消息队列
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										01. Kafka基础
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										02. 使用Kafka会考虑什么问题
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										08-MySQL
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										01. MySQL索引
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										02. MySQL事务和锁机制和MVCC
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										03. MySQL调优
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										09-项目场景类
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										01. 如何实现去重和幂等
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									<a href="/luffy/2021/08/20/09-%E9%A1%B9%E7%9B%AE%E5%9C%BA%E6%99%AF%E7%B1%BB/02.%20%E7%B3%BB%E7%BB%9F%E9%9C%80%E6%B1%82%E5%A4%9A%E5%8F%98%E5%A6%82%E4%BD%95%E8%AE%BE%E8%AE%A1/">
										02. 系统需求多变如何设计
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										03. 设计模式
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										10-计算机网络
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										01. HTTP
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										11-算法
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										01. 排序算法
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	01. List集合
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	<span>3y</span>
	<span>2021-08-19 15:01:38</span>
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	<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>要不今天来讲讲Java的List吧，你对List了解多少？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：List在Java里边是一个接口，常见的实现类有ArrayList和LinkedList，在开发中用得最多的是ArrayList</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：ArrayList的底层数据结构是数组，LinkedList底层数据结构是链表。</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtv4u6jk68j60lo09w3zk02.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>那Java本身就有数组了，为什么要用ArrayList呢？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：原生的数组会有一个特点：你在使用的时候必须要为它创建大小，而ArrayList不用。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在日常开发的时候，往往我们是不知道数组的大小的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：如果数组的大小指定多了，内存浪费；如果数组大小指定少了，装不下。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：假设我们给定数组的大小是10，要往这个数组里边填充元素，我们只能添加10个元素。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：而ArrayList不一样，ArrayList我们在使用的时候可以往里边添加20个，30个，甚至更多的元素</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：因为ArrayList是实现了动态扩容的</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtv4v39z3bj60mk090jsj02.jpg"></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：大概的意思就是：</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：当我们new ArrayList()的时候，默认会有一个空的Object数组，大小为0。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：当我们第一次add添加数据的时候，会给这个数组初始化一个大小，这个大小默认值为10</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：使用ArrayList在每一次add的时候，它都会先去计算这个数组够不够空间</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：如果空间是够的，那直接追加上去就好了。如果不够，那就得扩容</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>那怎么扩容？一次扩多少？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在源码里边，有个grow方法，每一次扩原来的1.5倍。比如说，初始化的值是10嘛。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：现在我第11个元素要进来了，发现这个数组的空间不够了，所以会扩到15</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：空间扩完容之后，会调用arraycopy来对数组进行拷贝</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtv4xru2o9j60mo07c74y02.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：哦，可以的。</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>那为什么你在前面提到，在日常开发中用得最多的是ArrayList呢？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：是由底层的数据结构来决定的，在日常开发中，遍历的需求比增删要多，即便是增删也是往往在List的尾部添加就OK了。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：像在尾部添加元素，ArrayList的时间复杂度也就O(1)</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：另外的是，ArrayList的增删底层调用的copyOf()被优化过</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：现代CPU对内存可以块操作，ArrayList的增删一点儿也不会比LinkedList慢</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtv52szl9bj60p4070gmg02.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>Vector你了解吗？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：嗯，Vector是底层结构是数组，一般现在我们已经很少用了。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：相对于ArrayList，它是线程安全的，在扩容的时候它是直接扩容两倍的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：比如现在有10个元素，要扩容的时候，就会将数组的大小增长到20</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtv53nezlij60mi08awff02.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>嗯，那如果我们不用Vector，线程安全的List还有什么？</strong>  </p>
<p><span style="color:#000080"><strong>候选者</strong></span>：首先，我们也可以用Collections来将ArrayList来包装一下，变成线程安全。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：但这肯定不是你想听的，对吧。在java.util.concurrent包下还有一个类，叫做CopyOnWriteArrayList</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：要讲CopyOnWriteArrayList之前，我还是想说说copy-on-write这个意思，下面我会简称为cow。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：比如说在Linux中，我们知道所有的进程都是init进程fork出来的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：除了进程号之外，fork出来的进程，默认跟父进程一模一样的。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：当使用了cow机制；子进程在被fork之后exec之前，两个进程用的是相同的内存空间的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：这意味着子进程的代码段、数据段、堆栈都是指向父进程的物理空间</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：当父子进程中有更改的行为发生时，再为子进程分配相应物理空间。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：这样做的好处就是，等到真正发生修改的时候，才去分配资源，可以减少分配或者复制大量资源时带来的瞬间延时。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：简单来说，就可以理解为我们的懒加载，或者说单例模式的懒汉式。等真正用到的时候再分配</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtv547k5wdj60qc08emxv02.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在文件系统中，其实也有cow的机制。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：文件系统的cow就是在修改数据的时候，不会直接在原来的数据位置上进行操作，而是重新找个位置修改。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：比如说：要修改数据块A的内容，先把A读出来，写到B块里面去。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：如果这时候断电了，原来A的内容还在。这样做的好处就是可以保证数据的完整性，瞬间挂掉了容易恢复。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：再回头来看CopyOnWriteArrayList吧，CopyOnWriteArrayList是一个线程安全的List，底层是通过复制数组的方式来实现的。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：我来说说它 的add()方法的实现吧</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：好</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在add()方法其实他会加lock锁，然后会复制出一个新的数组，往新的数组里边add真正的元素，最后把array的指向改变为新的数组</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：get()方法又或是size()方法只是获取array所指向的数组的元素或者大小。读不加锁，写加锁</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：可以发现的是，CopyOnWriteArrayList跟文件系统的COW机制是很像的</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtv57soszkj60mm0d8mzd02.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>那你能说说CopyOnWriteArrayList有什么缺点吗？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：很显然，CopyOnWriteArrayList是很耗费内存的，每次set()/add()都会复制一个数组出来</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：另外就是CopyOnWriteArrayList只能保证数据的最终一致性，不能保证数据的实时一致性。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：假设两个线程，线程A去读取CopyOnWriteArrayList的数据，还没读完</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：现在线程B把这个List给清空了，线程A此时还是可以把剩余的数据给读出来。</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtv5q8bjqej616g06qjsr02.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯，还可以。</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtm4126stpj61400midk402.jpg"></p>
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